AQUEOUS DROPLET SIZING BY INERTIAL CLASSIFICATION

The particle size distribution of an aerosol generated from an aqueous system is difficult to analyse because of the shrinkage of the drople ts due to solvent evaporation. These problems are very important for t he characterization of medical nebulizers, since most of the drugs del ivered via inhalation are water soluble. In situ methods for droplet s ize analysis, such as laser diffraction, phase Doppler anemometry and light scattering, do not determine either the initial or the equilibri um size distribution. With the residual technique, which means evapora ting the droplets and measuring the size and concentration of the resi duals, the instability of the aqueous droplets plays no role and the n ecessary radioactive labelling of the sprayed material allows a direct determination of the mass flow rate at the mouthpiece of the nebulize r. Tn this way it is possible to discriminate between the delivered dr ug solution and the water necessary to humidify the incoming air. The output of nebulizers of different designs is given for various operati ng conditions, filling volumes and solution concentrations. The measur ed droplet size distribution of a nebulizer is found to be fixed mainl y by its internal impaction system.